The copending application of A. Y. Feldblum et al. Ser. No. 08/024035, filed Mar. 1, 1993, now U.S. Pat. No. 5,286,338, hereby incorporated herein by reference, describes methods for making a microlens array on one flat surface of a silica substrate. The application points out that such arrays are useful, for example, for coupling light to and from optical fiber bundles of the type described in the Basavanhally, U.S. Pat. No. 5,135,590, granted Aug. 4, 1992.
According to the method, a matrix array of photoresist elements are defined on a silica substrate by photolithographic masking and etching. The photoresist elements are then melted to cause them to have curved or dome-shaped upper surfaces, and are thereafter solidified. The photoresist elements and the substrate are next subjected to reactive ion etching, that is, etching by a reactive gas in which the reaction is enhanced by applied radio frequency power. The photoresist elements cause differential etching in the substrate such that, after all of the photoresist has been etched away, the dome shapes of the original photoresist elements are replicated in the silica substrate.
After a microlens array has been made, I have found that it is difficult to align it with an optical fiber bundle such that each microlens is precisely properly aligned with the end of an optical fiber. Another problem is that, if the substrate thickness must always be smaller than the separation of the lens from the fiber, the substrate may be made so thin as to make the microlens array extremely fragile. Aligning optical fibers with lenses is also a problem in other photonics packages, such as those in which a laser, lens and fiber must be aligned, or in a detector in which a photodetector, lens and optical fiber must be mutually aligned.